An IP multimedia subsystem (IMS), which is a core session control layer of a fixed or mobile network, has become an issue discussed currently in the industry. Numerous IMS-related specifications have been defined in the 3rd Generation (3G) mobile communication systems and the TISPAN (Telecommunications and Internet Converged Services and Protocols for Advanced Networking) standards, including various aspects such as network architectures, interfaces, protocols, etc. Particularly, the security is an important aspect considered for the 3G and the TISPAN. According to the current specifications, an IMS network is divided into an access domain and a network domain in terms of the security, and security specifications are defined respectively for the access domain and the network domain. A security model of the IMS network is shown in FIG. 1.
A Call Session Control Function (CSCF) entity defined in the IMS network is adapted to accomplish such functions as control and routing during a call or session. P/S/I-CSCFs are distinguished from each other as per their different functions. The Proxy-Call Session Control Function (P-CSCF, Proxy-CSCF) is adapted for an access of a user equipment (UE), and any UE shall gain an access to the IMS network through the P-CSCF. The Service-Call Session Control Function (S-CSCF, Service-CSCF) provides such core functions as session control and routing. The interrogating-Call Session Control Function (I-CSCF, Interrogating-CSCF) is adapted for selection of the S-CSCF and the intercommunication between different service providers or different area networks, and is adapted to provide such functions as network masking. A Home Subscriber Server (HSS), which has evolved from a Home Location Register (HLR) and an Authentication Center (AUC), is adapted to store subscription data and configuration data of subscribers, and to support a function of Authentication & Authorization (AAA) for the subscribers.
As shown in FIG. 1, a security mechanism for the access domain includes two interfaces related to a User Equipment (UE): Interface 1 and Interface 2, where Interface 1 is a bidirectional authentication interface between the UE and an IMS network, and adapted to enable an subscriber authentication function, and Interface 2 is adapted to guarantee the communication security between the UE and the P-CSCF.
Interface 1 and Interface 2 are implemented in the 3GPP through application of an IMS AKA (Authentication, Key Authentication) mechanism during a registration process for the user equipment. Main network entities involved during the registration process of the user equipment include the user equipment UE, the P-CSCF, the S-CSCF and the HSS.
As shown in FIG. 2, the application of the IMS AKA mechanism during the registration process of the user equipment can be as follows.
1. An initial key K is shared between the UE and the HSS.
2. A subscriber initiates a registration request message SM1, and the S-CSCF requests data from the HSS through a message CM1. The HSS generates an authentication quintuple based on the initial key K and a sequence number SQN, and delivers the quintuple to the S-CSCF through a message CM2. The quintuple includes random data (RAND), an authentication token (AUTN), an expected response (XRES), an integrity key (IK) and a cipher key (CK).
3. The S-CSCF returns to the subscriber a 401 response (Authentication Challenge) carrying quadruple information except the XRES.
4. The P-CSCF stores information on the IK and the CK, and in the 401 response, sends information on the RAND and the AUTN to the UE.
5. The UE authenticates the credibility of the network equipment in accordance with information such as the initial key K and the SQN and the received AUTN issued from the network equipment. If the authentication succeeds, the network equipment is credible, and information on a response RES is generated according to the RAND and the K. The RES is taken as a “password” for calculation of a Response by the UE. A calculation result is sent to the network side in a message SM7 (Authentication Response). In the meantime, the UE calculates the IK and the CK itself.
6. The S-CSCF receives in a message SM9 the Response information generated according to the RES, and compares the information with the calculation result from the XRES. If both are identical, it can be determined that the authentication for the subscriber succeeds.
As can be seen from the above, the UE initiates the registration to the IMS network, and the bidirectional authentication between the UE and the IMS network is realized through the IMS AKA. Also, between the UE and the P-CSCF, a security alliance can be established, and both the cipher key CK and the integrity key IK can be shared. Both of the keys can be used to establish a secure communication channel between the UE and the P-CSCF.
For more information on the “IMS Access Domain Security”, reference can be made to 3GPP Technical Standard TS33.203, which gives detailed descriptions on the security of the IMS-network access domain, and for more information on the IMS AKA mechanism, reference can be made to TS33.203, Section 6.1 and IETF RFC3310.
In the wireless field, there are a large number of existing user equipments, which are inconsistent with the 3GPP protocol specifications and cannot support the access domain security mechanism as required for the 3GPP TS 33.203, such as a user equipment using a SIM card or a 2G user equipment using a USIM/ISIM card. In order to provide an IMS service for such an end subscriber, an access domain security mechanism referred to as “Early IMS” has been defined in TR 33.878. The basic principle for the Early IMS security access domain lies in that the application layer security can be enabled above the access layer security. After an equipment access has been authenticated at the access layer, the authenticated information is transferred to the application layer, which in turn performs an application layer security authentication for the subscriber request in accordance with such information. As shown in FIG. 3, the security mechanism for the Early IMS access domain can be divided into the following parts.
1. PDP Activation: a user equipment gains an access to a GPRS network through a Gateway GPRS Supporting Node (GGSN). During a Packet Data Protocol (PDP) activation process, the GGSN authenticates subscriber identities IMSI and MSISDN, and allocates a network transport layer identity (IP address) to the user equipment. The GGSN transfers correlations between the subscriber identities and the equipment IP address through a message “Accounting Request Start” to the HSS, which in turn stores the correlations.
2. Registration Request Authentication: the user equipment initiates a registration request REGISTER. When the P-CSCF forwards the request to the S-CSCF, a source IP address of the user equipment can be carried in the REGISTER request. In accordance with a public subscriber identity in the REGISTER request, the S-CSCF interrogates whether the user equipment has been registered. If it has not been registered, the equipment IP address corresponding to the public subscriber identity is obtained by the HSS through an MAR/MAA (the HSS statically configures the correlation between the public subscriber identity and the MSISDN, and thus the corresponding equipment IP address can be obtained through the public subscriber identity). The S-CSCF checks the equipment source IP address from the received REGISTER request, and if it is the same as that obtained from the HSS, the authentication succeeds.
3. Non-registration Request Authentication: since no security channel is established between the P-CSCF and the UE, the S-CSCF is required to authenticate all request messages initiated by the equipment, in order to ensure that the username be in correspondence with the source IP address. After the subscriber has been registered, the S-CSCF stores the correlations between the subscriber identities and the IP address. Upon receipt of any non-registration request message, the source IP address of the user equipment initiating the request has to be compared with the IP address stored in the S-CSCF for the subscriber, and if they are different, the request can be rejected.
In view of the above, the application of the Early IMS has the following limitation.
The GPRS access network can guarantee that the IP address of the user equipment will not be imitated by any other user, and thus, each of the equipments can only send a message with its own IP address.
The communication between the GPRS and the P-CSCF can be secured, and no NAT exists between the GPRS and the P-CSCF.
The simultaneous registrations for a single public subscriber identity of IMPU (IP Multimedia Public Identity) with respect to a plurality of private user identities of IMPI (IP Multimedia Private Identity) can not be supported.
Therefore, the security mechanism for the Early IMS access domain can be only directed to a specific wireless access environment, and also impose a special requirement on the access network. If a relevant user equipment has to be upgraded or adapted, the subscriber access security in any other access environment cannot be guaranteed.